The present invention relates to the field of cablelaying by forcing them into a conduit. It is particularly suitable for use in laying optical fiber telecommunication cables whose properties are likely to be detrimentally affected by the tension stresses required to pull the cable.
The conventional methods for laying cables, particularly in the individual pipes of multitubular conduits, are severely restricted as regards the lengths. The maximum distance is typically 300 m, 600 m in a few favorable instances. The factor limiting the pulling lengths is the friction between the outer cover of the cable and the wall of the conduit, which itself determines the pulling forces to be exerted at the end of the cable.
Numerous attempts have been made to reduce the friction between cable and conduit. Lubrication has been provided by placing a neutral grease between the conduit and the moving cable or by injecting an oil mist into the conduit before pulling. The pulling force has also been distributed along the cable and the force applied at the end of the cable correspondingly alleviated by propelling the cable by mechanical relaying means also called "pulling dogs", having friction drive rollers. Such pulling dogs are spaced along the path of the cable.
Such prior art approaches have drawbacks. Lubricating a conduit is a long and tiresome operation and the reduction of the friction coefficient is partly offset by adhesion of the cable to the conduit, due to the grease. The grease prevents the use of pulling dogs, for their rollers would slide on the cable. The latter drawback also exists in the case of oil mist lubrication. Pulling dogs can only be used for "dry" working and the use thereof is costly, since they are all the closer since the friction coefficient is increased due to dry working.
It is an object of the invention to provide a process for pulling cables, particularly optical fiber cables, into conduits which is improved as compared with the prior art methods; it is a more particular object to decrease the friction forces between the cable and conduit as the cable is pulled or otherwise moved along the conduit. It is still another object to authorize pulling large individual stretches of cable.
For that purpose, there is provided a method of locating a cable in a tubular conduit wherein, before the cable is pulled along said tubular conduit, a layer of balls having diameters substantially lower than the cable diameter is formed between the cooperating surfaces of the cable and the conduit and adhered to at least one of the cable and conduit to minimize the friction.
The word "adhered" should be construed as meaning that the balls are retained against the cable or conduit by forces which may be overcome, particularly for authorizing rolling friction or movement from one location to another. In other words, it does not mean that the balls are held fast or sticked by a process such as gluing which, after the glue has dried or cured, would not permit relative movement between the balls and the cable or conduit without permanently completely freeing the balls.
That method is a radical departure from the proposal in U.S. Pat. No. 4,232,981 to LEE, in which it is proposed to form a bed of balls or beads on part of the surface of a conduit. For that purpose, LEE suggests to flow a mixture of liquid and beads which are either more dense or less dense than the liquid for coating the lower or upper portion of the conduit. However, under practical conditions, a cable pulled along a conduit may rub on any portion of the surface of the conduit, particularly when the conduit is of relatively low diameter. That situation occurs when pulling optical fiber cables into multitubular conduits which typically have a diameter not higher than 100 mm. An additional limitation of LEE is that it is often impossible and always difficult to pump a liquid in a cable conduit. Last, most conduits are designed for not being immersed in water.
The forces to be exerted are much reduced as compared with conventional methods, since rolling friction replaces sliding friction; the length of the individual stretches of cable can be increased; the possibility of using pulling dogs is maintained, which may be more spaced than in the prior art techniques, which results in appreciable sparing: it suffices to remove the balls before passing the cable over the rollers of the pulling dogs, for example by means of brushes. In practice, the laying lengths are limited only by the capacity of the cable reels or the limitation in the manufacture of individual lengths of cable.
The balls used will generally be of synthetic material, typically polyamide or polyethylene. The diameter may vary within a fairly wide range. In practice, their diameter may be of from 0.1 mm (below which the defects of geometry or roughness of the conduits become preponderant) to a diameter of about 0.1 mm, beyond which difficulties are met for positioning the balls and forming a bed or carpet. In practice, the balls will generally not have a uniform diameter. The size distribution, which moreover the manufacture tolerances impose, seems beneficial: the smaller balls remain in the places of high friction whereas the larger ones are forced to the sides, which increases the angular zone of the cable carried by the balls. Classes of balls may further be provided, from which a choice will be made depending on the diameter of the cable to be pulled. By way of example, a class may be provided whose diameter is comprised between 0.1 (or better still 0.2) and 0.4 mm and a class of from 0.4 to 1 mm. The density will typically be of from 0.8 to 0.9.
Balls will be advantageously chosen whose diameter is in proportion to the diameter of the cable. For a cable having a diameter between 15 and 30 mm, the first class mentioned above may be used; for a cable having a diameter between 60 and 80 mm, the second above-defined class may be used. For intermediate diameters, balls may be used whose size distribution is of from 0.2 to 0.5 mm.
The balls may be positioned by a number of different processes. When the balls are of electrically insulating material, they may be caused to adhere to the outer cover of the cable as the latter advances by causing the cable to pass through a zone loaded with balls after electric charges have been deposited on the insulating cover of the cable. Adherence may also be caused by damping the cover. The carpet of balls may also be formed on the wall of the conduit by blowing balls as a suspension in air from one end of the conduit, blowing being carried out under a variable pressure, first high for propelling the balls into the part furthest from the injection point, then under reduced pressure. In most cases, the conduit wall is of dielectric electrically insulating material (such as PVC) and the balls will be retained against the wall of the conduit by electrostatic force.
As a general rule, the balls will be of a material having a hardness less than that of the outer cover of the cable and less than that of the conduit, to avoid damage resulting from their action.
As indicated above, the invention does not exclude the use of pulling relays. The laying may be carried out by pulling the endmost part of the cable and by driving the cable along by means of relays placed at distributed points spaced apart along the conduit. Then, the major part at least of the balls carried along by the cable is removed upstream of each relay and the balls are replaced or re-injected downstream of the relay.
The invention will be better understood from the following description of particular embodiments of the invention, given by way of examples only.